1. Field of the Invention
The present invention relates to an adaptive hybrid circuit for minimizing crosstalk from a transmitter to a receiver in the transceiver of a station when the transmitter and receiver are coupled to a two-wire transmission line or communication channel of a local area network comprised of a plurality of stations coupled to the same two-wire communication channel and, more particularly, to a control circuit for improving the operational capabilities of the adaptive hybrid circuit.
2. Description of the Prior Art
In the transmitter/receiver circuitry of a bus-type local area network based on CSMA/CD (carrier sense multiple access with collision detection), a hybrid circuit can be used for the separation or isolation of simultaneously transmitted and received signals at one of the stations in the network. This separation allows the implementation of collision detection at larger transmit to receive level ratios than without a hybrid. Thus, larger transmission distances can be achieved by using a hybrid. However, a hybrid must be designed for a fixed nominal transrission line impedance. This means that the maximum attenuation of the transmitted signal crosstalk into the received signal is obtained for only one transmission line impedance. In addition, the crosstalk attenuation for a given transmission line impedance will depend on the tolerances of the components that comprise the hybrid. Thus, any change in the impedance of the transmission line or in the values of the components in the hybrid, such as due to a temperature change, can prevent the hybrid from optimally cancelling the transmitted signal crosstalk from the received signal.
The above-cited U.S. patent application Ser. No. 623,302 provides, in a transceiver of a local station in a local area network, a unique adaptive hybrid circuit for adaptively cancelling crosstalk from a transmitter to a receiver. This unique adaptive hybrid circuit is comprised of a comparator, an integrator and an adjustment circuit. The output from the receiver is compared in the comparator with a transmit signal to develop a first signal which causes the integrator to develop a second signal having an amplitude which varies as a function of the state of the first signal. The transmitted signal, the second signal and a composite signal comprised of the received signal and a crosstalk signal from the transmitter are applied to the adjustment circuit. In the adjustment circuit, the transmitted signal is multiplied by the second signal to develop a crosstalk cancelling signal which varies automatically to cancel the crosstalk signal in the composite signal for any given transmission line impedance. When the crosstalk is cancelled, the output of the adjustment circuit only contains the received signal which is detected by the receiver.
In the operation of the adaptive hybrid circuit of the above-cited U.S. patent application Ser. No. 623,302, two operational problems may occur.
1. A first operational problem may occur as a result of a collision during the initial part of any subsequent transmission after the completion of a training mode. During the training mode of operation, the transceiver of the local station adjusts (trains) its adaptive hybrid circuit to minimize any crosstalk from the transmitter to the receiver of the transceiver. After the completion of the training mode, a normal mode of operation is entered. During this normal mode of operation the transceiver continues to adjust the adaptive hybrid circuit during each subsequent transmission. A collision (simultaneous attempt to transmit) with another station may occur in the initial part of a subsequent transmission. Because some period of time is needed to detect and react to the collision and to disable the adaptive hybrid circuit, the adjustment of the adaptive hybrid circuit may be disrupted by that collision.
2. A second operational problem may occur as a result of an undesired change in the adjustment of the adaptive hybrid circuit between two consecutive transmissions. Between two consecutive transmissions the adjustment of the adaptive hybrid circuit is latched such that the hybrid control voltage does not change between transmissions. However, any of a number of events may occur to cause an undesired change in the setting of the adjustment of the adaptive hybrid circuit between transmissions. Such an undesired change may be caused by: (a) a noise spike that changes the count of the counter used in the integrator of the adaptive hybrid circuit, (b) a large variation in the power supply voltage that changes the adjustment of the adaptive hybrid circuit or (c) a variation in one of the components in the adaptive hybrid circuit due to shock or vibration. As a result of one or more of the above causes of an undesired change in the adjustment of the adaptive hybrid circuit, the adaptive hybrid circuit may become misadjusted such that at the start of the next transmission crosstalk occurs between the transmitter and receiver of the transceiver. This misadjustment of the adaptive hybrid circuit may be large enough to result in a false collision detection which will disable the adjustment of the adaptive hybrid circuit and cause the adaptive hybrid circuit to remain misadjusted. Repeated attempts to transmit will produce the same result, thereby producing a false collision detect deadlock situation from which the adaptive hybrid does not automatically recover.